"In thermonuclear weapons, radiation from a fission explosive can be contained
and used to transfer energy to compress and ignite a physically separate
component containing thermonuclear fuel."
The 3 basic concepts of thermonuclear devices,
U.S. DOE, Sept 1980, Duane Sewell,
Assistant Secretary of Energy for Defense,
Official Declassification Act.
FUSION PRINCIPLES
Under solar conditions (high temps of about 100 millions degrees C, 1 milion
megabars pressure), H atoms fuse into He. Three isotopes of H exist:
H1 (P) protium
H2 (D) deuterium
H3 (T) tritium.
Protium reacts too slowly even in the sun so deuterium and tritium are used.
Under solar conditions, the H atoms gain enough kinetic enery to overcome
the electrostatic repulsion of their positive charges. The electrons which
are normally found surrounding H nuclei have already been ionised. You have
a plasma of positive nuclei. He is formed in a H-H reaction, releasing energy.
Sources of D and T
Heavy water (D2O) is present at 1 part in 6700 in normal tap water. You can
separate the heavy water, and then obtain deuterium gas. D2 gas
is obtained via electrolysis.
Tritium is radioactive, and is obtained via bombardment of Li6 with thermal
(slow) neutrons. It beta decays like: T -> He3 + e
T fuses with D at a temperature an order of mag lower than for D-D fusion,
hence its usefulness in a weapon.
Lithium
The lightest of metals, only 1/2 as dense as water. Found combined with
other elements in igneous rocks and mineral spring water. Li7 is separated
electrolytically from Li7Cl. Has several isotopes: Li5 to Li9. Li6 and Li7
are used in weapons, and are naturally occuring. Li5, Li8, and Li9 are
man-made radioisotopes.
Li6 is present as 7.5% of all naturally occuring Li. Separation methods
include electrolysis, distillation, chemical exchange, or EM methods. Li
bonds with H to form the solid Li6D.
Back to the Story
Since the mass of the resultant He is less than the mass of the separate H,
the excess energy is converted into radiation and kinetic energy of neutrons.
The energy of these fast neutrons is high enough to split normal U-238. Slow
neutrons only transmute U-238 into Np-239 (which then beta decays into
Pu-239).
One cubic metre of gaseous deuterium, when fused into helium, yields the
equivalent of about 10 megatons of TNT.
Deuterium and tritium are gases at room temperature, so their storage in a
weapon would be cumbersome. Instead, a substance called lithium deuteride
(Li6D or Li7D) is used. This material has the property of being a whitish,
slightly-blue powdery light salt-solid (which is extremely hygroscopic) at
room temperature. It is made by heating metal lithium in a vessel, into
which deuterium gas is injected. It is then pressed and shaped into a
ceramic.
When a neutron is absorbed by a LiD molecule, the molecule breaks up into
a He, H3, and a deuterium. The D can then reacts with the T in fusion. This
releases enormous amounts of energy, much greater than you would get in
a fission reaction. The end products include a free n, and a He. Schematically:
U-238 fission releases fast neutrons and heat (thermal kinetic energy of
neutrons).
Li6 + n -> He4 + T + 4.7 MeV
then
D + T -> He4 + n + 17.6 MeV.
n + U-238 -> neutrons + fission products + energy
These reactions occur in under 1/10-6 secs. Additional reactions are:
Li6 + D -> 2(He4) + 22.4 MeV
Li6 -> 2(He4) + n
Li6 + P -> He4 + He3 + 4.0 MeV
Li7 + P -> 2(He4) + 17.3 MeV
Li7 + D -> Li8 + P
Li7 + n -> He4 + T + n
D + Li7 -> Be8 + n + 15.1 MeV
D + D -> T + H + 4.0 MeV
D + D -> He3 + n + 3.25 MeV
D + D -> He4 + 23 MeV
T + T -> He4 + 2n + 12.2 MeV
He3 + D -> He4 + H + 18.3 MeV
D + n -> T
Beryllium is useful in the core of a fission mass since you can use it to
increase the neutron flux:
Be9 + n -> Be8 + 2n
Be9 + D -> Be8 + T + 4.53 MeV
For a thermonuclear reaction, you have to compress the Li6D solid to 15-30
times it's original uncompressed density at RTP (15lbs/foot^3). Compression
is needed to:
(1) increase fusion *probability*. You pack the molecules closer together.
In the process, you pave the way to overcoming the electrostatic repulsion
of the H atoms in the Li6D.
(2) increase fusion *rate*, since you get quicker reactions when the reactants
are packed closely together than far apart. The *time* for a reaction is
inversely proportional to fuel density. Denser fuels mean shorter reaction
times, and hence more chance of a larger number of reactions. The *rate* of
reaction, on the other hand, is proportional to the square of the fuel density.
Increase the density by a factor of 30, and your rate increases by a factor
of 900.
Compression is a form of inertial confinement fusion (ICF). You are in effect
counteracting the explosive forces released in the fusion, by giving the
reactants an inwardly directed momentum. So the whole mass of fuel stays
together. It's collapsing in on itself; at the same time it wants to tear
itself apart.
PS, 1994